I just got a used Macbook Air with 128 GB drive. It had Big Sur, but after reset it's back at Mojave. So this OS must have been saved on the disk [Edit: no, it was probably downloaded]. Will I save storage space if I create a bootable USB-stick with Big Sur, format the disk and perform a clean install, instead of upgrading?
I doubt the end result would make enough difference to be worth the effort.
You'd increase usage during the upgrade process as opposed to clean & install, but by the time it's finished, both methods will end up with the same result.
The other advantage of updating would be if you'd already set up your iCloud login etc.
You may, btw, have got Mojave because of the recovery method used to clean install -
- Cmd ⌘ R
would reinstall the latest macOS that was installed on your Mac, without upgrading to a later version
- Opt ⌥ Cmd ⌘ R
would upgrade to the latest macOS that is compatible with your Mac
- Shift ⇧ Opt ⌥ Cmd ⌘ R
would reinstall the macOS that came with your Mac, or the version nearest to it that is still available.